Zebra mussels threaten ancient site

Underwater forest may be destroyed

December 11, 2002|By Julie Deardorff, Tribune staff reporter.

Captivated by tales of an ancient underwater forest, Jack Fessenden, a student at Northeastern Illinois University, recently dove through the dark, frigid waters of Lake Michigan to see the 8,300-year-old tree stumps for himself.

What Fessenden, 32, saw has alarmed Great Lakes researchers: Thick layers of zebra mussels are now caked over the wooden relics, threatening to destroy the site 15 miles northeast of Calumet Harbor.

Scientists fear that the threadlike structures zebra mussels use to stay in place will tear apart the ancient wood within a few decades.

The recent infestation has implications that reach far beyond losing the submerged oak, ash and hickory trees, which have been preserved in 85 feet of water for thousands of years. Clues to geological history could be lost as well, said Michael Chrzastowski of the Illinois State Geological Survey.

Since the collection of more than 50 tree stumps was discovered in 1989 by salvage operator Alan Olson, researchers have searched for similar drowned forests in southern Lake Michigan. Additional sites would help confirm theories about lake water levels and shed light on a relatively unknown period of time when the lake levels were very low.

"If those [undiscovered] sites are being colonized by zebra mussels, they may be destroyed before we ever discover them," said Chrzastowski, a senior coastal geologist who is collecting sediment samples of the area. "It's very disappointing. It may eliminate our ability to do more research on the history of the lake level rise, based on finding more tree stumps."

The zebra mussels could also destroy old wooden shipwrecks, another window into the past.

"We certainly had a lot of trouble with zebra mussels and water intake, water plants and the fouling of boat bottoms, but we hadn't thought much about the impact on archeological sites on the Great Lakes," said Northeastern Illinois earth science professor Charles Shabica, who has been studying the underwater forest since it was discovered.

"There has been an immense amount of degradation in the wood since we first saw the structures," Shabica said.

The original stumps pulled up for carbon dating and species identification showed small colonies of zebra mussels, about 15 to 20 per relic. The stumps Fessenden and three other divers saw Nov. 23 had at least 150 zebra mussels stacked on top of each other.

"It was like looking at a ball of zebra mussels," said Fessenden, an electrician from Grayslake who studies environmental science part time. "You had to know what was down there to know it was a tree stump."

An invasive species that reached U.S. waters in the mid-1980s by riding in the ballast water of ships, zebra mussels have caused trouble by clogging water intake pipes, suffocating native freshwater clams and altering the lakes' freshwater ecosystem. Since their unwelcome arrival in the Great Lakes, zebra mussels have caused losses of up to $5 billion, according to the U.S. Environmental Protection Agency.

As filter feeders, the dime-size mollusks vacuum up nearly every microscopic aquatic plant or animal they can find. They attach to all living and non-living surfaces and even each other, eventually forming dense colonies up to a foot thick.

Though the zebra mussels don't eat the wood, they cause damage because they use strong threads to attach to hard surfaces. When they die, they eventually tumble off, ripping off pieces of the tree in the process.

"My guess is that in about 20 years those stumps will be reduced to a mere ghost of what they were," Shabica said.

Based on the tree stumps, researchers believe that 8,300 years ago, the level of Lake Michigan was 85 feet lower than it is today. When the water level rose, it killed the trees.

The location of the stumps in a river valley had something to do with why they were preserved, Chrzastowski said. Once the dead trees are sticking above water, the wood rots and waves break off the branches.

"But being in a river valley, they were drowned and underwater before the actual lake line got there," Chrzastowski said.